The present invention relates to tires for motorcycles, passenger cars, trucks, and buses which, as compared with conventional ones, generate less noise when they come into contact with the road.
The tread of conventional tires has a pattern consisting of design elements or unit blocks which continuously repeat in the circumferential direction of the tire. This tread pattern causes air in the groove of the tread to be compressed and relieved as the tire turns. The repeated compression and relief of air form the compressional wave of air which makes audible pattern noise or impacting noise. This noise becomes amplified when it resonates with some parts of the vehicle. It is known that this kind of noise can be reduced when the noise frequency is distributed over a broad range and the resonance is prevented. This is accomplished by changing the tread design such that the repeating consitituent units are arranged at varied pitches. This is known as the variable pitch method. According to this method, the design elements are arranged in the circumferential direction of the tire at several different pitches, so that the time interval of pulse noise or vibration is changed and noise of a specific frequency is not generated when the tire comes into contact with the road and leaves the road. However, the variable pitch method is not necessarily satisfactory. Even though the variable pitches for the design elements are properly dispersed according to the frequency modulation theory, they do not reduce the sound pressure energy per se. Moreover, if the difference between the longest pitch and the shortest pitch is excessively great, the individual design elements differ in stiffness. This, in turn, adversely affects the vibration characteristics of the rotating tire and causes uneven wear and abrasion. It is also known that if the variable pitches are arranged regularly in a specific manner, the tire generates noise offensive to the ear.